He changed his personal name to "Tadahisa" upon coming of age, and later changed his surnname to "Hata" after marrying the daughter of Koremune Hironobu, who was a descendant of the prestigious Hata clan.

During the Gempei War, he sided with his father in the battle against the Taira clan.

In the aftermath of the war, he was initially named the jitō ("constable") of Shioda shōen ("estate") in Shinano province in central Japan. Then in 1187 he was named shugo ("military governor") of Satsuma province at the southernmost tip of the southernmost Japanese island of Kyushu. He sent initially sent a retainer, Honda Sadachika, to take possession of the province in his name while he accompanied Yoritomo on his invasion of Mutsu province in 1189. He finally went to Satsuma personally in 1196, conquered Hyūga and Ōsumi provinces, and established his headquarters at Shimazu estate in Hyūga, which name he adopted as his new surname.